MessagePort: message event

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨September 2015⁩.

Note: This feature is available in Web Workers.

The message event is fired on a MessagePort object when a message arrives on that channel.

This event is not cancellable and does not bubble.

Syntax

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("message", (event) => { })
onmessage = (event) => { }

Event type

A MessageEvent. Inherits from Event.

Event properties

This interface also inherits properties from its parent, Event.

MessageEvent.data Read only: The data sent by the message emitter.
MessageEvent.origin Read only: A string representing the origin of the message emitter.
MessageEvent.lastEventId Read only: A string representing a unique ID for the event.
MessageEvent.source Read only: A MessageEventSource (which can be a WindowProxy, MessagePort, or ServiceWorker object) representing the message emitter.
MessageEvent.ports Read only: An array containing all MessagePort objects sent with the message, in order.

Examples

Suppose a script creates a MessageChannel and sends one of the ports to a different browsing context, such as another <iframe>, using code like this:

const channel = new MessageChannel();
const myPort = channel.port1;
const targetFrame = window.top.frames[1];
const targetOrigin = "https://example.org";
const messageControl = document.querySelector("#message");
const channelMessageButton = document.querySelector("#channel-message");
channelMessageButton.addEventListener("click", () => {
 myPort.postMessage(messageControl.value);
});
targetFrame.postMessage("init", targetOrigin, [channel.port2]);

The target can receive the port and start listening for messages and message errors on it using code like this:

window.addEventListener("message", (event) => {
 const myPort = event.ports[0];
 myPort.addEventListener("message", (event) => {
 received.textContent = event.data;
 });
 myPort.addEventListener("messageerror", (event) => {
 console.error(event.data);
 });
 myPort.start();
});

Note that the listener must call MessagePort.start() before any messages will be delivered to this port. This is only needed when using the addEventListener() method: if the receiver uses onmessage instead, start() is called implicitly:

window.addEventListener("message", (event) => {
 const myPort = event.ports[0];
 myPort.onmessage = (event) => {
 received.textContent = event.data;
 };
 myPort.onmessageerror = (event) => {
 console.error(event.data);
 };
});